Study On Properties Of Composite Magnesium Oxysulfate Cement Reinforced By Metal Cations

With the rapid development of industry,CO₂ and other greenhouse gases are emitted in large quantities,among which the CO₂ emission of ordinary Portland cement accounts for 8%of the global greenhouse gas emissions.Therefore,it is very important to find alternative green cementing materials.Magnesium oxysulfate cement（MOSC）as green cementing material is formed by light-burned magnesia powder（LBM）and a certain concentration magnesium sulfate solution with the advantages of light weight,wear resistance and high temperature resistance etc.Compared with ordinary Portland cement,the calcination temperature required for LBM powder is lower,and the CO₂emission of preparation of MOSC products accounts for about 50%-60%of the emission of preparation of ordinary Portland cement,so MOSC is regarded as green cementing material.However,due to the poor mechanical properties of MOSC,limiting MOSC to use in civil engineering,construction and other industries.To improve the mechanical properties of MOSC,this subject firstly uses ferric sulfate（FS）,copper sulfate（CS）and aluminum sulfate（AS）as additives to MOSC.The effect of FS,CS and AS on the setting time,mobility,compressive/flexural strength,volume stability,phase composition,microstructure and pore structure of MOSC under the condition of air curing were analyzed.Additionally,to determine the curing temperature range suitable for curing MOSC,the effect of curing temperature on the hydration process,compressive strength and phase composition of MOSC with different metal cations were studied.Results show that:FS,CS and AS optimized the pore structure of MOSC and reduced the total porosity,increasing the matrix density,improving the compressive/flexural strength and reducing the volume shrinkage rate of MOSC by promoting the formation of Mg/Fe solid solution,Mg/Cu solid solution and M-（Al）-S-H gel in the system.The order of improving effect of these three modifiers on compressive strength was Cu²⁺>Fe³⁺>Al³⁺,and the order of improving effect on flexural strength and volume stability was Fe³⁺>Cu²⁺>Al³⁺.The curing temperature between 20℃-25℃is more conducive to the growth and development of 5Mg（OH）₂·Mg SO₄·7H₂O in MOSC system,thus improving the compressive strength of MOSC.The MOSC with 3 wt.%FS,0.5 wt.%CS and 1 wt.%AS had the highest compressive strength at 25℃,25℃and 20℃,respectively.The 28d compressive strength was 91.1 MPa,96.1 MPa and 87.4 MPa,respectively.When the curing temperature reaches 30℃,the content of Mg（OH）₂ in MOSC system increases to 17%-22%,and the content of 517 phase decreases to 39%-45%,resulting in the reduction of the compressive strength of MOSC.